Darwin: Evolution as descent with modification

  • Descent with modification was Darwin’s theory on how the Tree of Life was created


  • Darwin accounted for extinction and inheritance but did not know about genes or DNA


  • The mechanism proposed by Darwin was different from Lamarck

What does ‘Descent with modification’ really mean?





Evolution: Change in the genetic

composition of a population over

generational time

How would you define a population?

Population = group of individuals of same species, coexisting in the same time and place, with the potential of interbreeding

What does change in genetic composition mean?

Change in DNA nucleotide sequence (genotype) that leads to a alternate version of a trait (phenotype)

At least 7 genetically distinct lineages of giraffes exist

Generational Time determines the pace of evolution





Human = ~27 years




Bacteria = ?



Remember: Generations are the timescale for inheritance from parent to offspring

Mechanism of Darwinian Evolution = Natural Selection

  • Focuses on adaptation through natural selection (not use/disuse)
    • characteristics that enhance fitness in an environment
    • these characteristics must be heritable
  • Different populations of organisms live in different habitats, so they gradually accumulate changes to be better suited to that habitat ( adaptation)


?Proposed that new species could arise from an ancestral form with enough adaptations

Adaptation viewed through homology

Homology =

Evolution by natural selection rules (1)

(1) Individuals in a population vary in their traits


A trait is an observable and measurable characteristic of an organisms phenotype


Trait variation comes from random mutations in the genetic code

Evolution by natural selection rules (2)



(2) Some of these differences are heritable


Evolution by natural selection rules (3)

(3) In each generation, many more offspring are produced than can survive


  • Only some will survive to reproduce
  • Some will produce more offspring than others

Evolution by natural selection rules (4)

(4) Individuals with certain heritable traits are more likely to survive and reproduce ( adaptation)


  • Individuals with certain traits produce more offspring than those without those traits ( selection)
  • Individuals are selected naturally, by the environment
  • Favorable traits will accumulate in a population over generations

Natural Selection acts on individuals but populations evolve


  • Populations are the unit of evolution
    • INDIVIDUALS DO NOT EVOLVE (remember Skee Lo)


  • E.g. Drought caused adaptation in beaks of finches on the Galapagos Islands
    • selection pressures from food availability (seeds) changed as certain plants suffered from drought


  • Average beak depth increases over time because deep-beaked individuals had greater reproductive success
    • short-beaked finches did not adapt longer beaks during drought!

Evolution happens: Antibiotic resistance

Modern history of the evolution of antibiotic resistant bacteria


  • Penicillin used in 1943 to stop S.aureus
    • by 1945 20% of S.aureus resistant
    • due to new enzyme ’penicilinase”


  • Methicillin used in 1959 to stop S.aureus
    • deactivates production of cell walls
    • resistant strains emerged within 2 years
    • evolved a different enzyme to make cell walls


  • Natural selection then lead to spread of MRSA
    • Methicillin-resistant S.aureus super bugs!
    • MRSA now resistant to multiple antibiotics

Antibiotic resistance is not new to evolution!



  • Antibiotic resistance is ancient and widespread
    • genes to protect against penicillin, tetracycline, etc.
    • soil bacteria are rich in resistance


  • Multi-drug resistance found in bacteria isolated from Lechuguilla Cave
    • ecosystem isolated from surface for 4 million years
    • Paenibacillus cave isolate is resistant to most clinical antibiotics
    • 18 resistance mechanisms, including 3 unknown!

Evolution can occur rapidly in certain organisms

Why generational time matters…


If you were a bacteria today, in 2024….


  • You started class as a sexually reproducing adult and leave welcoming your great-grandchildren


  • Yesterday…
    • Start of Classic Period for Maya
    • China’s Three Kingdoms reunified under Emperor Jin
    • Warrior Queen Zenobia breaks with Roman Empire
    • Pyramid of the Sun and Moon completed
    • Birth of Prophet Mohammed

Generational time and evolution

Humans = ~25 years
Bacteria = ~20 minutes

















How long is the average course of antibiotic treatment?
Do you now better understand new variants of SARS-CoV-2?